#1 is perfect.
#2 = do you see 3 names there? No way can you use the "yo" form to talk about all 3 of you! Look also at the adjective "alegres" which is also plural.
#3 = either you are speaking to Mario about both of you, or again there are 3 people involved.
#4 = the dog = WE are in the park? No.
#5 = incorrect because está requires the accent mark.
#6 = you got one!
#7 = the chairs = IT? No, that is plural and do not forget the required accent mark!
#8 = correct
#9 = correct if your textbook teaches vosotros/vosotras.
#10 = you are not talking TO Ana because there is no comma for direct address, so you are talking ABOUT Ana.
#11 = it IS plural but you are talking about 2 GIRLS: contentas
#12 = Marco (the subject) is singular, the verb is singular; how could he adjective be PLURAL? No.
#13 = note "los perros" = masculine plural, NOT feminine. No.
#14, #15, #16 , #17 = OK
Next time please don't skip so many lines. Skipping ONE line is perfect, or 2, but no more. It takes way too long for me to slide up, down, up, down.
#18 = la Argentina
#19, #20 = OK